Robotically (as used herein, the terms “robot” or “robotically” and the like include teleoperation or telerobotic aspects) controlled instruments are well known and often used in minimally invasive medical procedures. FIG. 1 shows an example of an instrument 100 having a structure that is simplified to illustrate basic working principles of some current robotically controlled medical instruments. Instrument 100 includes a tool or end effector 110 at the distal end of a shaft 120. The proximal end of shaft 120 attaches to a transmission or drive mechanism 130 that is sometimes referred to as backend mechanism 130. During a medical procedure, end effector 110 and the distal end of shaft 120 can be inserted through a small incision or a natural orifice of a patient to position end effector 110 at a work site in the patient. End effector 110 as illustrated includes jaws 112 and 114 that may be used at the work site for clamping, gripping, cutting, or other purposes. Other types of end effectors, for example, scalpels, wire loops, and cauterizing instruments, are known and could alternatively be mounted on the distal end of shaft 120. Normally, a surgical instrument would further include a wrist mechanism (not shown) at the distal end of shaft 120 to provide additional degrees of freedom of motion for positioning, orienting, and using end effector 110.
Tendons 121 and 122, which may be stranded cables, rods, tubes, or similar structures, run from backend mechanism 130 through shaft 120 and attach to end effector 110. A typical surgical instrument would also include additional tendons (not shown) that connect backend mechanism 130 to other structural members of end effector 110 or of a wrist mechanism, so that backend mechanism 130 can manipulate the tendons to operate end effector 110 and/or the wrist mechanism when performing the desired procedure at the work site. FIG. 1 illustrates two tendons 121 and 122 attached to jaw 112 in a pin joint structure, where jaw 112 is mounted for rotation about a pivot pin 116. To enable both clockwise and counterclockwise rotations of jaw 112, tendon 121 acts on jaw 112 at a moment arm about pivot pin 116 such that pulling on tendon 121 causes a torque tending to rotate jaw 112 clockwise in the view of FIG. 1. Similarly, tendon 122 acts at a moment arm such that pulling on tendon 122 causes a torque tending to rotate jaw 112 counterclockwise in the view of FIG. 1. Jaw 112 is thus provided with bi-directional actuation through pulling in a length of one tendon 121 or 122 and simultaneously releasing an identical length of the other tendon 122 or 121. Mechanisms other than pin joints are known or can be devised that provide bi-directional actuation of a distal joint through pulling in a length of one tendon 121 or 122 and releasing an equal and opposite length of the other tendon 122 or 121. For example, U.S. Pat. No. 6,817,974 (filed Jun. 28, 2002) entitled “Surgical Tool Having Positively Positionable Tendon-Actuated Multi-Disk Wrist Support” by Cooper et al. and U.S. Pat. No. 6,394,998 (filed Sep. 17, 1999) entitled “Surgical Tools For Use In Minimally Invasive Telesurgical Applications” by Wallace et al., both of which are incorporated herein by reference, describe some known medical instrument structures in which actuation requires pulling one or more tendons while releasing lengths of one or more other tendons.
Slack in tendons 121 and 122 can cause malfunctions, for example, by permitting tendons 121 and 122 to derail from guides or pulleys (not shown) that route tendons 121 and 122 through instrument 100. Slack can also cause jumpy or unpredictable motion of the instrument. To avoid creating slack in tendon 121 or 122 when moving jaw 112, backend mechanism 130 operates to release a length of one tendon 121 or 122 while simultaneously reeling in an equal length of the other tendon 122 or 121. Tendons 121 and 122 can be attached to the same capstan 132 but wrapped in opposite directions to provide the desired movements of tendons 121 and 122 when a drive motor (not shown) turns capstan 132. In cases where an end effector has several degrees of freedom that are controlled by several tendons, for instance as described in patent U.S. Pat. Nos. 6,394,998 and 6,817,974, the backend mechanism can include mechanisms other than capstans to perform the function of releasing and reeling in related lengths of tendons in order to avoid slack in the tendons as distal joints are turned. It can be seen that tendons 121 and 122 can be two separate components, or they may be part of a closed loop component with a capstan actuator, such as that disclosed in U.S. Pat. No. 7,316,681 B2 (filed Oct. 4, 2005) entitled “Articulated Surgical Instrument For Performing Minimally Invasive Surgery With Enhanced Dexterity And Sensitivity” by Madhani et al., which is incorporated herein by reference.
Avoiding slack may also require tendons 121 and 122 to be pre-tensioned, particularly when end effector 110 may be used for pushing and pulling, clamping, gripping, or other actions that encounter resistance. In particular, backend mechanism 130 can apply tension to tendon 122 to cause jaw 112 to clamp down on material between jaws 112 and 114. Increasing tension in tendon 122 causes higher clamping force but also causes tendon 122 to stretch. To prevent the stretching of tendon 122 from causing a corresponding amount of slack in tendon 121, tendon 121 can be preloaded with a tension that stretches tendon 121. For example, assuming tendons 121 and 122 are identical, tendon 121 can be pre-loaded with a tension about equal to or greater than maximum clamping tension used in tendon 122. As a result, tendon 121 starts stretched by the preloaded tension, and when applying a clamping force, the stretching of tendon 122 reduces the tension in tendon 121, allowing tendon 121 to contract without becoming slack.
The tensions preloaded in tendons can increase the forces that a backend mechanism must apply to operate an instrument. In particular, tension increases friction where tendons may ride on guides or solid surfaces. Also, if shaft 120 is a flexible tube such as might be employed to follow a natural lumen in a patient's body, the preloaded tension causes friction where tendons 121 and 122 contact curved surfaces of shaft 120. In all the above cases, increased friction quickly makes accurate control of the motion of end effector 110 difficult and can lead to imprecise manipulation of, e.g., tissue, sutures, and needles during a surgical procedure. Further, preloading of the correct tension in tendons of a medical instrument can also increase the complexity of the manufacturing process of the medical instrument. Accordingly, medical instruments are desired that can avoid slack in drive tendons without requiring high preloaded tension.